This invention relates to electric power supplies, and particularly to a switching power supply capable of a.c.-to-d.c. voltage conversion, featuring provisions for attainment of closer approximation of the input current waveform to a sinusoidal wave, and for that of a higher power factor, than by the comparable prior art.
The switching power supply or voltage regulator has long been familiar which comprises a rectifying and smoothing circuit to be coupled to a source of a.c. power, and a d.c.-to-d.c. converter circuit connected to the rectifying and smoothing circuit. The rectifying and smoothing circuit comprises a rectifier circuit and a smoothing capacitor. Although so simple in configuration, this known rectifying and smoothing circuit possesses the disadvantage of a somewhat poor power factor as a result of the fact that the smoothing capacitor is charged only at or adjacent the peaks of the a.c. voltage of sinusoidal waveform. Another drawback is that the input current is not favorable in waveform.
Designed to defeat these shortcomings, a more advanced switching power supply has also been suggested which comprises an inductor connected between the rectifier circuit and the smoothing capacitor, and a switch which is connected between the pair of outputs of the rectifier circuit and which is controllable via the inductor. The smoothing capacitor is connected in parallel with the switch via the rectifying diode. This known circuit comprising the inductor and the switch is sometimes referred to as the step-up power-factor improvement circuit. As the switch is turned on and off at a repetition frequency higher than the frequency of the input a.c. voltage, the current flowing through the inductor has a peak value in proportion with the instantaneous value of the input a.c. voltage. The results are a close approximation of the input current waveform to a sinusoidal waveform, and an improvement in power factor. It is also possible to make the voltage across the smoothing capacitor higher than the maximum value of the a.c. voltage.
It has also been known and practiced to connect a capacitor in parallel with the switch for on-off control of the rectifier output voltage, in order to protect this switch from overcurrent and to lessen its noise production. The capacitor will be charged when the switch is off, thereby preventing a rapid voltage buildup across the switch. However, when the switch is turned on, the energy that has been stored on the capacitor will be released through the switch, with consequent power loss.
An additional problem arises when a d.c.-to-d.c. converter circuit is coupled to the aforesaid step-up power-factor improvement circuit, the latter being then used as d.c. power supply. Including a switch for on-off control of the d.c. voltage, the d.c.-to-d.c. converter circuit provides another source of switching loss. A provision of separate circuits for on-off control of the switch in the power-factor improvement circuit and that in the d.c.-to-d.c. converter circuit, and for zero-voltage turning-on of both switches, would make the complete power supply system too complex in construction and expensive of manufacture.
It must also be taken into consideration that the on-off control of the power-factor improvement circuit switch and d.c.-to-d.c. converter circuit switch at the same repetition frequency is undesirable. Noise might then be produced, or the switches might become unstable in operation, as a result of the frequency interference of both switches.
Japanese Unexamined Patent Publication No. 8-154379 suggests a different type of switching power supply. The switch in the d.c.-to-d.c. converter circuit is utilized for switching both the d.c. voltage across the smoothing capacitor and the current through the inductor for power factor improvement. One switch performs the dual purpose of power factor improvement and d.c.-to-d.c. conversion, but to lesser extents than by two switches.
The present invention has it among its objects, in a step-up power supply of the type defined, to reduce power loss and noise production due to switching by simpler means than heretofore and without the difficulties so far experienced.
Another object of the invention is to further improve the power factor of the power supply and, at the same time, to most effectively and inexpensively lessen power losses due to the switch included in the power-factor improvement circuit and that in the d.c.-to-d.c. converter.
Briefly, the present invention may be summarized as a switching power supply capable of translating a.c. voltage of sinusoidal waveform into d.c. voltage. Included is a rectifier circuit connected to a pair of input terminals for rectifying the input a.c. voltage, the rectifier circuit having a first and a second output for providing a rectifier output voltage. A main switch is connected to the first output of the rectifier circuit via a main inductor on one hand and, on the other hand, to the second output of the rectifier circuit. The main switch has capacitance means for its soft switching, the capacitance means being in the form of either a discrete capacitor connected in parallel therewith or its parasitic capacitance. A rectifying diode is connected to the rectifier circuit via the main inductor. A smoothing capacitor is connected in parallel with the main switch via the rectifying diode. An ancillary inductor is connected to an input terminal or an output terminal of the main inductor and electromagnetically coupled thereto. An ancillary switch is connected to the input terminal or the output terminal of the main inductor via the ancillary inductor on one hand and, on the other hand, to the second output of the rectifier circuit. A first reverse-blocking diode is connected in series with the ancillary inductor. Also included is a switch control circuit which is connected to the primary switch for on-off control thereof at a repetition frequency higher than the frequency of the input a.c. voltage, and to the ancillary switch for on-off control thereof at such a repetition frequency, and with such conducting periods, as to assure soft turn-on of the main switch.
The invention as summarized above features the ancillary inductor electromagnetically coupled to the main inductor, and the ancillary switch connected in series with the ancillary inductor. Voltage will be impressed to the ancillary inductor when the ancillary switch is turned on earlier than the main switch. This ancillary inductor voltage will act to cause a rapid decrease in the magnitude of the current charging the smoothing capacitor due to the energy stored on the main inductor. When the rectifying diode, connected to the smoothing capacitor for charging the same, becomes nonconductive, the soft-switching capacitor connected in parallel with the main switch will release the energy that has been stored thereon. The main switch may be turned on after the current charging the smoothing capacitor has dropped to zero and the soft-switching capacitor has completed its energy release. The main switch will then be turned on at zero or very low voltage, and at zero or very low current. Thus is accomplished the so-called soft switching of the main switch.
The soft-switching capacitor and the ancillary inductor constitute in combination a resonant circuit conducive to the soft switching of the main switch. As a result, less power loss and less noise occur at the main switch, and the power factor is improved with the power loss kept at a minimum.
Preferably, the switching power supply according to the invention additionally comprises a transformer, a rectifying and smoothing circuit connected to the transformer for providing output d.c. voltage, a second main switch connected to the smoothing capacitor via the transformer, second soft-switching capacitance means such as a capacitor connected in parallel with the second main switch, a second ancillary inductor electromagnetically coupled to a primary winding of the transformer and having one extremity connected to a junction between the second main switch and the smoothing capacitor, and a second ancillary switch connected to another extremity of the second ancillary inductor. Besides being connected to the first recited main switch for its on-off control, the switch control circuit is connected to the second main switch for on-off control thereof in order to cause d.c. voltage to be intermittently applied from the smoothing capacitor to the primary winding of the transformer, and to the first and the second ancillary switch for on-off control thereof at such a repetition frequency, and with such conducting periods, as to assure soft turn-on of both first and second main switches.
The second soft-switching capacitance means and the second ancillary inductor constitute in combination a second resonant circuit conducive to the soft switching of the second main switch. This second main switch is therefore also kept from power loss and noise production, and d.c.-to-d.c. conversion is accomplished with minimal power loss.
The first ancillary switch is designed for control of the first resonant circuit comprised of the first soft-switching capacitance means and the first ancillary inductor, and the second ancillary switch for control of the second resonant circuit comprised of the second soft-switching capacitance means and the second ancillary inductor. Both first and second ancillary switches are drivable by the same signal from the same switch control circuit as for both main switches.
The electromagnetic coupling of the first ancillary inductor with the main inductor is effective to restrict the current flowing through the main inductor into the smoothing capacitor when the first ancillary switch is turned on with consequent voltage application to the first ancillary inductor. The current thus flowing into the smoothing capacitor drops to zero in a relatively short period of time, resulting in early commencement of discharge by the first soft-switching capacitance means.
Similarly, as a result of the electromagnetic coupling of the second ancillary inductor with the transformer primary, the energy release from transformer primary to rectifying and smoothing circuit is completed in a relatively short period of time when the second ancillary switch is turned on with consequent voltage application to the second ancillary inductor. The result is an early commencement of discharge by the second soft-switching capacitance means.
Electromagnetically coupled together, moreover, the main inductor and the first ancillary inductor are manufacturable as a compact, integral combinations. The same applies to the transformer primary and the second ancillary inductor which are electromagnetically coupled together. The transformer is manufacturable in compact, integral combination with the second ancillary inductor.
It is also an advantage of this invention that the two main switches are both driven at the same switching frequency. The switch control circuit is much simpler and inexpensive in construction than if the main switches are driven at different frequencies. The driving of the main switches at different frequencies is objectionable for an additional reason: The different driving frequencies would be difficult of creation because of their possible mutual interference. Noise production would also be easier to occur.
In some embodiments of the invention, not only are the two main switches driven at the same frequency, but they are turned on simultaneously. The switch control circuit can then be made further simpler in construction.
In some other embodiments, however, the two main switches are turned on at different moments. One such embodiment employs what are termed conducting period limitation signals for variously delaying the beginnings of predefined tentative conducting periods of the main switches. The conducting period limitation signals permit the main switches to be turned on at different moments that are independently adjustable.
A yet further embodiment is disclosed in which the main switches are turned on when the voltages across them drop below predetermined reference voltages. The main switches can then be turned on at zero or very low voltages more positively than in cases where they are turned on at moments by timers.
The above and other objects, features and advantages of this invention will become more apparent, and the invention itself will best be understood, from a study of the following detailed description and appended claims, with reference had to the attached drawings showing the preferred embodiments of the invention.